Anthozoa

Sea Anemones: a brief
note:

Sea Anemones belong to the
phylum of animals known as the Cnidaria,
from the cnida or sting capsule that are present in this major group
of animals that also include the corals, jellyfish, hydroids, medusae,
and sea fans. Sea anemones, corals and their allies form the class
known as the Anthozoa.

Structure and Form

Sea anemones are primitive
animals
consisting mostly of a column with a single opening, the mouth, used to
ingest food and expel wastes. Stinging organelles are found throughout
the animal, in the tentacles, and in some species in stinging threads (acontia),
and beads (acrorhagi).

Tentacles are found in groups
of six in many families, with the largest usually on the inside, with increasing
numbers near the parapet of the column. The flattish area on top around
the mouth is called the disc, or oral disc. The upright column can vary
in shape. Wart-like protuberances called verrucae are noticeable
in rows on the column in some species. Other anemones possess small suckers,
but these need to be observed under a magnifying glass. Anemones are attached
to rocks by the basal disc.

Anemone bodies are
divided into two primary layers known as the
ectoderm (or epidermis)
for the exterior, and the internal
endoderm (or gastrodermis) connected
by the mesoglea tissue. The internal gastric centre is also known as the
cavity. Beadlet Anemones,
Actinia
equina, are common on all rocky coasts around Britain.

Movement

Sea Anemones often remain
in same place for several days, weeks or even months. However they can
move around in two ways:

They creep around on their
basal disc, so slowly that the movement can only be ascertained by a change
of position in aquaria, or observed by time lapse photography.

They inflate and let the
tides and currents take them into a new location. This phenomenon is seen
in aquaria when conditions become unfavourable,

e.g. it is observed
in the Dahlia Anemone, Urticina
felina, when the temperature in the tank exceeds 22o
C.

Predators

Sea Anemones are probably
distasteful to most fish and are ignored by crabs in British seas. However,
the Grey Sea Slug, Aeolidia papillosa,
feeds exclusively on sea anemones, and the shallow water fish, the Tompot
Blenny,
Parablennius gattorugine, demonstrates an eagerness
to decimate an anemone population in aquaria as the first choice of food.

Grey Sea Slug
feeding on a Beadlet Anemone.

Photograph by Andy Horton.

Feeding

Sea Anemones are not plants
but predatory animals. The sting capsule in the tentacles and other parts
of the sea anemone each contain a coiled hollow filament, usually barbed
and containing venom. They are used to catch smaller organisms, for defence,
and to fight territorial battles. These capsules can be seen under the
microscope and their mechanism is arguable the fastest action in the whole
of the animal kingdom. When triggered by touch, the capsule turns inside
out and drives the filament into its prey discharging its venom.

The tentacles manipulate
the food to the single opening for digestion. Wastes are expelled through
the same opening. Sea anemones breathe through
the general body surface. There are no special organs.

A Functional
Biology of Sea Anemones 1991JM ShickSeries: FUNCTIONAL
BIOLOGY SERIESThe first comprehensive
treatment of the sea anemones for fifty years, this book presents an integrated
synthesis of their biology.It emphasizes their physiological
ecology, evolutionary biology and biological interactions with other taxa,
and it includes a wide-ranging bibliography.